1. Field of the Invention
The present invention relates to an electrophotographic toner. More specifically, the invention relates to an electrophotographic toner used as a developer in a so-called electrophotographic image forming apparatus such as an electrostatic copying machine or a laser beam printer.
2. Description of the Related Art
Image formation in the electrophotographic image forming apparatus is conducted in accordance with each of the steps of charging, exposure, development, transfer, and fixing. At first, in the charging step, the surface of a photoreceptor as an image support for forming static latent images is charged uniformly. In the exposing step, a light in accordance with image information is irradiated to the charged surface of the photoreceptor thereby forming static latent images on the surface thereof. In the developing step, a black toner, for example, is deposited selectively to the formed static latent images, and visible images (toner images) with the toner are formed on the surface of the photoreceptor. Then, in the transferring step, the toner images are transferred by electrostatic force on a transfer paper. In the final fixing step, the toner images transferred on the transfer paper are melted by heating to fix the toner images on the transfer paper.
Further, electrophotographic coloring techniques have been developed rapidly in recent years, and full color image forming apparatus have been developed and provided to markets. The markets for the color image forming apparatus have been extended along with popularization of black and white image forming apparatus. Generally, for reproducing colors in the full color image forming apparatus, toners of three colors of yellow (Y), magenta (M), and cyan (C) as three primary colors in subtractive color mixing, or four color with addition of black (K) to the three colors are used. As the procedure for reproducing the colors, the steps up to the charging, exposure, development and transfer are repeated among the image forming steps to each of the colors of C, M, Y and K and toner images comprising toners of a plurality of colors are superposed on the transfer paper thereby forming full color images. Then, in the final fixing step, the superposed toner images are melted and fixed on the transfer paper. In the procedures, since the superposed toner images are mixed by melting, colors are reproduced in accordance with the principle of subtractive color mixing.
In the full color electrophotography described above, since it is necessary for conducting development for a plurality of times and superposing several kinds of toner images of different colors on one identical support in the fixing step, charging characteristic and the fixing characteristic to be provided for the toners of respective colors are extremely important factors.
That is, in order to keep stable and good color reproducibility of full color images, it is at first necessary to transfer a predetermined amount of a toner on the transfer paper from the developing step in the transfer. The deposition amount of the toner in the developing and transferring steps greatly depends on the charging characteristics of the toner such as the rising characteristic of the charged amount, the circumstantial stability and the aging stability for the charged amount, the durability, etc of the toner. Therefore, a charge controller is incorporated in the toner to keep a good chargeability as a first measure. A toner using a polyester resin is usually negatively charging and, as a negatively charging controller, oil soluble dyes such as oil black and spiron black, metal-containing azo dyes, metal naphthenate salts, metal alkyl salicylate salts, fatty acid soaps, resinic acid soaps, etc. are used.
Then, various devices have been made for finely dispersing the charge controller uniformly in the toner. In addition, it is also an important factor to select the kind and the chemical composition of the binder resin for the toner. In order to make the charging characteristic of the toner excellent, it is an important factor not only using a charge controller of excellent charging characteristic but also selecting and using a resin capable of finely dispersing the charge controller uniformly in accordance with the charge controller to be used.
Each of the color toners of different colors transferred to the transfer paper is fixed in the fixing step. In this case, the fixed color toner has to be such a color toner having a wide range of color reproducibility that can be melted till the boundary between each of toner particles is eliminated to obtain transparency. Further, it requires appropriate glossiness.
As the fixing device for fixing the color toner, a heating roller utilizing a material of excellent surface releasability relative to the toner is used. However, in most of the devices, a great amount of oil is coated on the surface of the fixing roller, etc. in order to prevent offset. Accordingly, the transfer paper is contaminated with the oil, and provision of the oil supply portion and an oil coating portion is essential, which results in a problem such as increase in the size of the fixing device, complication of the mechanism and, further, increase in the cost.
Generally, the reason of using the oil for fixing the color toner is as described below. Compared with fixing under heating of a black toner for use in usual black and white printing, it is necessary to increase the fusibility to obtain a transparency and reduce the viscosity of the color toner. However, in such a color toner, cohesion during melting is lowered to cause adherence of the toner to the fixing roller to result in high temperature offset phenomenon. Accordingly, in order to prevent the high temperature offset, a great amount of oil was coated to the fixing roller thereby decreasing the adherence of the toner to be fixing roller.
In view of the above, with an aim of simplifying the fixing device and preventing undesired effects of the oil on the images (oil contamination, stickiness), it has become necessary to cope with the development of an oilless mechanism of saving a silicon oil coating mechanism. For this purpose, improvement for the fixing characteristic with the toner in a wide range from low temperature to high temperature has been desired so that the fixing roller can be used without coating the oil and provision of a so-called oilless toner for coping therewith has been attempted.
For this purpose, it has been proposed to disperse a wax in the toner. As the wax, amide wax, carnauba wax, higher fatty acids and esters thereof, higher fatty acid metal soaps, partially saponifying higher fatty acid esters, higher aliphatic alcohols, polyolefin waxes, and paraffin waxes are used. It is important that the wax exudes properly from the molten toner. It is considered that exuding of the wax can prevent adherence of the molten toner to the surface of the fixing roller thereby suppressing the offset phenomenon.
Further, as a second measure, in the toner, external additives for improving and controlling the charged amount and the fluidity are added to a matrix particle incorporated with the binder resin, the colorant, the charge controller, etc.
For the binder resin for use in the full color toner, those resins excellent in the sharp melting property, the coloring property and the fixing property are demanded and polyester resins are used usually. Then, the polyester resins usually show strong negatively charging characteristic and the toner using such resins is designed for negative charging and, as the negative charge controller used for the toner, oil soluble dyes such as oil black and spiron black, metal containing azo dyes, metal naphthenate salts, metal alkyl salicylate salts, fatty acid soaps, and resinic acid soaps, etc. are used.
The external additive includes, for example, fine inorganic powder such as of silicon dioxide, titanium oxide, aluminum oxide, cerium oxide, zinc oxide, tin oxide, and zirconium oxide, those formed by applying a surface treatment to them by using a hydrophobic treating agent such as silicon oil, and silane coupling agent, as well as fine resin powder such as of polystyrene, acrylate, styrene-acrylate, polyester, polyolefin, cellulose, polyurethane, benzoguanamine, melamine, nylon, silicon, phenol, and vinylidene fluoride.
For making the chargeability of the toner excellent, it has been known that uniform and fine dispersion of the charge controller in the toner particle or on the surface of the toner particle is at first important. In a case where the charge controller is agglomerated to a portion in the toner or is not finely particulated even if it is dispersed uniformly and present as coarse particles, initial charged amount can not be obtained. Further, in a case where the charge controller is not dispersed, the rising characteristic of charging is poor or a toner with remarkable decay of charging is formed. Accordingly, various devices have been conducted for uniformly and finely dispersing the charge controller in the toner, and selection for the kind and the chemical composition of the binder resin is an important factor. Accordingly, in order to make the charging characteristic of the toner excellent, it is important not only using a charge controller of excellent charging characteristic but also selecting and using a binder resin capable of uniformly and finely dispersing the charge controller in accordance with the charge controller to be used.
Then, as the external additive for improving the chargeability, the fluidity, the adhesion, or the like, a fine powder of an inorganic oxide is generally added, that is, fine silica particles are generally added to toner matrix particles, to obtain a toner. However, in a case of fine silica particles used usually, while they are particularly excellent in the effect of improving the toner fluidity, they have a strongly negative polarity and, particularly, in a low temperature and low humidity circumstance, they excessively increase charges of the negatively charging toner.
Further, under high temperature and high humidity, since they absorb water content to decrease the chargeability, it results in a great difference of the chargeability between the low temperature and low humidity condition and high temperature and high humidity condition, so that the transportability of the toner to the developer support and the chargeability thereof can not be optimized both for high temperature and high humidity and low temperature and low humidity conditions to result in the problem of failure in the reproducibility of image density, fogging, dropping of toner and, further, contamination inside the machine.
With an aim of improving them, fine silica particles applied with a surface treatment by an amino compound for moderating the effect of negative charging are used. However, no satisfactory effect of reducing the difference of the charged amount depending on the circumstance can be obtained only by the use of fine silica particles and no sufficient effect is obtained in a case of using a polyester resin as the binder resin in the toner matrix particles. In a case where a sufficient chargeability can be provided under high temperature and high humidity, the charged amount increases excessively under low temperature and low humidity and the distribution of charging is widened remarkably and, particularly, lowering of the developability and increase of fogging are ferocious.
Accordingly, as a method of eliminating the circumstantial fluctuation of the toner particle characteristics, a method of applying a surface treatment with a hydrophobic treating agent such as a silicone oil or a silane coupling agent is utilized.
As described above, even when a hydrophobic treatment, or a treatment of moderating negatively charging property is applied to fine silica particles, dependence of charging on the circumstance, charging rate and the poor distribution of charges inherent to silica have not yet been improved at present.
In view of the above, external addition of inorganic compounds other than the fine silica particles to the toner has been studied and titanium oxide or the like has become used. Titanium oxide has a low charging level and has a merit that the charging level and the circumstantial dependence can be controlled easily by using a treating agent. However, titanium oxide is generally obtained by a method of taking out titanium oxide crystals using ilmenite ores by a sulfuric acid method and, since titanium oxide is obtained by heating and baking after purification by a wet method, and chemical bonds formed by dehydrating condensation reaction are naturally present in titanium oxide, strong agglomerates are present therein. Such agglomerated particles can not be easily re-dispersed by the existent method. That is, crystalline titanium oxide obtained as a fine powder contains secondary agglomerates and tertiary agglomerates and the effect of improving the fluidity of the toner was extremely lower compared with that of silica.
Further, a demand for improving the image quality has been increased, particularly, color photography, etc., and it has been attempted to attain high image quality by making the particle size of the toner smaller but refining of the toner matrix particles increases the inter-particle adhesion and lowers the fluidity of the toner further. The phenomenon tends to occur, particularly, in titanium oxide.
Accordingly, it has been known to use and add hydrophobic titanium oxide and hydrophobic silica together to toner particles in order to make the improvement of the fluidity and the circumstantial dependence of charging compatible for the toner.
With regard to the first measure, for overcoming the problem in the charging characteristic and the fixing characteristic of the toner, a technique as described, for example, in Japanese Unexamined Patent Publication JP-A 2004-4207 has been proposed. JP-A 2004-4207 proposes a technique of incorporating an organic bentonite as a charge controller in a polyester resin having an aliphatic alcohol ingredient as a binder resin, thereby enhancing uniform and fine dispersibility of the charge controller in the toner to improve the charging characteristic of the toner. Further, it discloses to use, particularly, those waxes containing higher fatty acid ester compounds and/or aliphatic alcohol compounds to the polyester resin having the aliphatic alcohol ingredient as a releasing agent. Improvement of the charging characteristic and the fixing characteristic have been intended by using them.
Since polyester resin having the aliphatic alcohol ingredient used as the binder resin disclosed in JP-A 2004-4207 is inferior in the humidity resistant circumstantial characteristic to the polyester resins having aromatic alcohol ingredient, it is expected that no sufficient charging stability can be obtained. Therefore, as images are formed repetitively, it gives rise to a problem such as contamination of images or contamination inside the machine by toner scattering, etc.
Further, the wax containing the higher fatty acid ester compound and/or aliphatic alcohol compound is highly compatible with the polyester resin having the aliphatic alcohol ingredient and has good dispersibility. However, the wax is poor in view of the exuding effect from the toner and no satisfactory offset resistance and fixing strength can be expected. The anti-offset characteristic and the fixing characteristic are extremely important factors.
Further, with regard to the second measure, JP-A 2004-4207 discloses therein to use silica and titanium oxide subjected to a hydrophobic treatment as an external additive. It is disclosed that the charging characteristics, etc. can thus be improved, images with no background contamination can be obtained even in high speed development, toner scattering inside the developing apparatus can be decreased and stable images can be obtained from the initial state to the completion of development.
By the way, JP-A 2004-4207 neither discloses nor suggests the optimization of the external additive for charge control and it mentions nothing at all about the stability of the image quality due to circumstantial change or the like or suppression of change for the image density.